11 Mar 2010

Sub-9kHz Induction Field DX?

With all the discussion recently on the LF Reflector about how to radiate a signal on sub-9kHz I'm left wondrering how far, practically, one could expect to get simply with sub-9kHz electromagnetic induction using multi-turn loops at each end.

It's quite easy for anyone to generate 50-100W or so and feed this into a largish diameter multi-turn loop. Correctly aligned and with a similar loop at the far end and using decent weak signal processing just what range might be possible using modes like QRSS or WSPR? Has anyone tried this to see just what is practical, simply using induction fields?

The Cave Radio Group (CREG) may have some answers but I'm not a subscriber so can't read any of their documents.

LATER: Jim Moritz M0BMU did some sums (see my webpage on sub-9kHz radio) and reckons that about 4.4kms is the best one could do with 100W and a 10m diameter loop and purely inductive communications. More power and bigger (low loss) loops could get a bit further still.

8 comments:

Anonymous said...

Hello Roger

What would be the minimum distance between conductors for Earth
mode communication at a VLF frequncey of 9kHz. Would you be able to conduct experiments from the average back garden.

Tony

Roger G3XBM said...

My experiments some years ago were with a 10m TX earth rod separation with one at the house and one at the bottom of the garden. I could detect a keyed 1kHz signal at 0.3kms (possibly aided by pipe work) but also at 0.2kms by ear in the middle of a field well away from any pipes or cables. The RX base spacing was around 5m maximum. With narrowband modes like WSPR and QRSS I would imagine that at least double the range would be possible. Remember this was using just 4W from a TDA2002 audio chip.

Anonymous said...

Roger
I have just down loaded the Pdf article at
the bottom of your VLF page. Transmission
test were caried out with the earth conduction
mode and the RX set up used a Datong VLF
Converter with a small active recieving loop.
If you set up your Tx to transmit a WSPR signal
It should be possible to recieve your signal over
much greater distance.
Your original test distance of 0.3km might be
multiplied by a factor of one hundred.

KR Tony

Roger G3XBM said...

The increase in range will be much smaller Tony because the attenuation for conduction and induction fields is much greater than for a radiated signal (inverse cubed rather than inversed squared). Nonetheless I would expect WSPR and QRSS to help increase the range a reasonable degree - maybe 2-3 time the distance. I shall be finding out shortly when I dust off the earth mode kit and connect it to WSPR software at each end of the link.

Anonymous said...

I have not done any expeimenting at VLF
I have played around with various transmitters
Into dummy loads.
I have got very little experience at such low
frequencies.
If you were to use greater output powers
You would not acheive any significant ncrease in distance
due to the inverse cube law of attenuation.

Tony

Anonymous said...

Roger

Are there any more links and articles on earth conduction mode you could sugest for the VLF novices like myself.

KR Tony

Roger G3XBM said...

Most are on my webpage Tony. See http://sites.google.com/site/g3xbmqrp/Home/10khz. There were a few NATO AGARD papers dating from the 1960s but I've not found these on the net. They are also a bit heavy on maths!

Anonymous said...

Thank you for all the help and tips on this interresting
subject.

Tony